Quote Originally Posted by Kirk Keyes View Post
H2SO3 is dissolved SO2. If the pH is too low, the SO2 is driven out of solution and it's stinky. (I'm on a stinky roll these days.) If the pH is higher, then the SO2 stays in solution as the sulfite ion, SO3--. The SO3-- ion is a good oxygen scavenger, which is one of its main uses in developers, and it is converted to sulfate, SO4--.

In other words, metabisulfite quickly becomes sulfite in solution. If the pH is high enough, there's no free SO2 gas dissolved in the solution. If the solution is acidic enough, you get back the SO2 gas from the dissolved sulfite and you can still have some sulfite ion in solution, but there's not some sort of "metabisulfite" ion present at any time in the solution.
So we have a free SO3-- oxygen scavenger in the very early Rodinal type developer Perfect because this helps prevent Dichroic fog and maybe why Anti-foggants needed to be added when they switched to using Potassium Sulphite.

Rodinal is working at "Beutler" like levels of concentration, so the SO3-- is likely to be more critical.

Next point is obviously as the SO3-- is converted in localised areas to sulphate it will harden the emulsion . . . . . . .

So we have a bit of logic for the three distinct phases of Rodinal that we know of:

1 Primitive what we've just discussed "The Classic Formulae"

2 Middle age - ? & when they switched from Metabisulphite to Sulphite, now we see Antifoggant and Bromide needed possibly for the edge effects - Still made by Calbe

3 Agfa/A&O current Rodinal, less p-Aminophenol, higher pH - 14 more bromide etc

That's my theory anyway